The present invention relates generally to an improved mechanically stabilized thermally conductive interface pad for transferring thermal energy from a heat generating semiconductor device to a heat dissipator such as a heat sink or heat spreader. More specifically, the present invention relates to such an interface which comprises a laminate pad with upper and lower thermally conductive laminae positioned upon and extending through apertures from opposed surfaces of an open mesh grid. In other words, portions of each of the opposed laminae extend through apertures in the stabilizing mesh grid to form a continuum, thus enhancing both mechanical stability and heat transfer efficiency without the creation of additional thermal interfaces.
The upper and lower laminae preferably comprise formulations of highly thermally conductive polymer compounds such as a polymeric matrix loaded or filled with solid particulate and a liquid or low melting point metal. It will be understood that the term “liquid metal” is intended to refer to a metal or alloy having a melting point which is generally below about 90° C., although some higher melting materials may be included in the definition. The solid particulate typically forms clusters which become coated with the low melting metal and are distributed throughout the matrix. The arrangement of the present invention further provides for extrusion or oozing of portions of the loaded polymer laminae through the grid apertures. Extrusion through these apertures is achieved through application of heat and pressure to a laminae/mesh grid preform which causes the opposed laminae surfaces to merge and create a continuum of the loaded polymer. Accordingly, the continuum creates a mechanically stabilized composite thermally conductive pad which is formed without creation of additional large thermal barriers or interfaces extending along and across the thermal path. As a result, enhanced mechanical stability is achieved without sacrificing valuable properties such as high thermal conductivity and mechanical compliance.
Liquid metals as well as thermally conductive particulate have each been proposed for incorporation in a polymeric matrix to form thermally conductive interface pads. In the past however, the application of liquid metals for this purpose had not been widely used, primarily because of problems created instability and/or tendency of the liquid metal to oxidize or form alloys and amalgams, thereby altering and modifying the physical properties of the liquid metal component in the pad. In certain arrangements, the liquid metal component would become oxidized, both along the surface as well as in the bulk structure. By way of example, dispersed liquid metal droplets had a tendency to coalesce, a process known as Ostwald ripening, and cause macroscopic separation of the metal from the polymer matrix. In addition the oxidation of the liquid metal was sometimes accelerated upon exposure to warm and/or humid environments; this leading to the formation of brittle oxides which reduced the effectiveness of the thermal properties of the compound. Finally, while some highly thermally conductive components of prior art devices are typically electrically conductive, this property may not always be undesirable. Instability of liquid metals was believed due at least in part to the extremely high surface tension and other chemical and physical properties of the metallic component.